The present invention relates generally to conductivity sensors, and more specifically to conductivity sensors suitable for detecting relatively low conductivity.
Fuel cells are increasingly being investigated as an alternative power source for automotive vehicles. Fuel cells generate electricity using a membrane. The process also generates heat. Fuel cells therefore must be cooled by circulating coolant therethrough. In automotive applications, the coolant may be a mixture of ethylene glycol and water. Fuel cells, however, are sensitive to the conductivity of the coolant. If the conductivity is too high, the coolant should be changed to allow the fuel cell to operate efficiently. Therefore, a means for sensing the conductivity to allow a warning to the vehicle operator so that the fluid may be changed is necessary.
Known configurations for conductivity sensors include multiple electrodes that are used to sense the conductivity therebetween. Known sensors are unable to adequately detect low conductivities. Another drawback to providing a conductivity sensor is the space within a fuel cell vehicle is scarce. Therefore, a small conductivity sensor should be provided so that the package size of the vehicle does not need to be increased. Another problem with fuel cell vehicles is that the fuel cell compartment is an electrically noisy environment. This can cause erroneous readings using known electrical conductivity sensors.
It would therefore be desirable to provide a conductivity sensor that has a small package size, allows detection of low conductivity fluids and is capable of performing in an electronically noisy environment.
The present invention provides an improved conductivity sensor particularly suited for low conductivity detection. The present invention includes a conductivity sensor having a first annular electrode having a first inner diameter and a second annular electrode having the first inner diameter. A tubular portion is positioned axially between the first electrode and the second electrode. The tubular portion has a second inner diameter greater than the first inner diameter between the first electrode and the second electrode.
In a further aspect of the invention, a method of assembling a conductivity sensor comprises:
coupling a first annular electrode having a first inner diameter to a tubular portion;
coupling a second annular electrode having the first inner diameter to the tubular portion so that the tubular portion positioned axially between said first electrode and said second electrode.
One advantage of the invention is that the sensor and control circuit according to the present invention is capable of measuring extremely low conductivity in fluids. Another advantage of the invention is that the sensor may be coupled within a fluid path and therefore take very little space. Yet another advantage of the invention is that because a synchronous detector is used in the control circuit, the circuit is not prone to errors due to electrical noise.
Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.